A fuel tank for a motorcycle is required to be light and strong, to be leak proof, and to be easily produced. The fuel tank of this kind according to the prior art is constructed such that a mounting rear stay is welded to the back of an inner member or such that both outer and inner members are rearwardly extended to provide an extension acting as the rear stay. According to the former construction, however, there is a problem in strength and maintenance because the rear stay is welded in a cantilever construction to the inner member which is made of a relatively thin steel plate. The solution of this problem is accompanied by a disadvantage that the aforementioned intention to make the tank light has to be sacrificed. According to the latter construction, on the other hand, the above disadvantage can be eliminated, but there arise other disadvantages because it is difficult to register the outer and inner members at their rear stay portions and because the seam welding operation cannot be ensured due to the leakage of an electric current, i.e., ineffective shunt during the seam welding process.